Why, oh why, are you torturing yourself and your wife with this? Your insecurity is the issue, not your wife’s behavior. As your wife has told you, you can’t blame her for being attractive to someone. She has nothing to be sorry for. She can’t “make him” stop hitting on her – no matter how much respect she has for you. It’s important for you to look at why you aren’t satisfied with being the lucky “winner” for 21 years. I worry that your need to win a senseless argument may result in the very thing you fear. You’re now flirting with someone to make a point. Your wife probably feels disrespected and distrusted. I suppose you could ask her to stop talking to this guy completely but at this point her sense of pride may make that difficult.
I strongly urge you to get into some couples therapy to resolve this once and for all. You want to spend your remaining years together without this old story lurking in the background and threatening your peace. Deal with it so you can just relax and enjoy the rewards of a long marriage.
I wish you well.